The present invention relates to a method and relative device for automatically aligning a workpiece, in particular a pack of printed circuit boards, on a machine tool table.
As is known, a workpiece must be positioned accurately on the worktable of a machine tool, which is achieved by means of locating elements normally provided on the workpiece itself. In the case of packs of printed circuit boards, each pack features a pair of pins projecting from one face of the pack, and which are fitted inside seats on a clamping fixture fitted to the table. The fixture comprises a locating member for clamping one of the pins, and a pair of bars for gripping the other pin.
Machining printed circuits calls for a high degree of precision. For example, each hole drilled on a drilling machine is about 0.5 mm in diameter and must be centered exactly on the respective circuit pad, which may be about 0.6 mm in diameter. As such, the circuit must be positioned up to one micron, and the same degree of precision must be guaranteed over the entire surface of the circuit, which is normally rectangular with sides measuring many centimeters in length.
The positioning precision of the pack clamping fixture depends on how accurately the fixture and the fixture connection to the table are machined, and on how accurately the two pins are fitted to the pack. And, since the drilling position is determined by the machine on the basis of two coordinate axes referred to the table, the coordinate axes of the drilling positions must coincide with those of the table for the pack to be positioned accurately.
Even minor mechanical inaccuracies on the fixture, the fixture connection and/or the pins, may result in linear or angular deviation of the coordinate axes of the pack with respect to those of the table, thus resulting in unacceptably inaccurate drilling. Known fixtures therefore have the drawback, on the one hand, of having to be machined and assembled extremely accurately, and, on the other, of being incapable of correcting inevitable inaccuracies in machining and/or assembly.